fn main() {
    let mut s1 = String::from("foo");
    let s2 = "bar";
    s1.push_str(s2);
    println!("s1 is {}, s2 is {}", s1, s2);

    let s3 = s1 + &s2;  // 注意s1被移动了，不能继续使用
    println!("s3 is {}", s3);

    let mut s = String::new();
    s.push_str("lo");
    s.push('l');
    println!("s is {}", s);
}
